This invention relates to a bistable electromagnetic relay, and in particular to a novel bistable electromagnetic relay requiring low power consumption and in particular embodiments providing means for ensuring the connection in the event of accidental opening of the relay.
Electrical relays are devices that enable a connection to be made between two electrodes in order to transmit a current. Conventional electrical relays are based on the electromagnetic effects. In a conventional relay the ON (ie closed) state is obtained by applying a current to an electromagnet so that an electromagnetic attraction exists between the poles of two magnets and this attraction acts to bring electrodes into contact to make an electrical connection. Conventionally this ON state is maintained until the current to the electromagnet is removed, the magnetic poles then lose their attractive power and are moved away from each other by a biasing spring that normally urges them apart thus opening the contact between the electrodes. Such conventional relays have been very well-known for many years and are very effective. However, they do suffer from the drawback that generally the current must be continually supplied to the electromagnet in order to maintain the ON condition and this does result in relatively high power consumption. In order to overcome this problem a number of designs for bistable relays have been proposed
U.S. Pat. No. 4,703,293 and U.S. Pat. No. 4,975,666 are illustrative of examples of what are known as xe2x80x9cpolarized relaysxe2x80x9d and U.S. Pat. No. 5,867,081 is an example of a xe2x80x9cbistable relayxe2x80x9d that use a permanent magnet that maintains the attractive force even when the current to the electromagnet is switched off. However, in these designs because the ON state is maintained by the permanent magnet, it is difficult to achieve a reliable switch from the ON (closed) to OFF (open) state when that is desired. Furthermore, if the relay is being maintained in the ON state solely by the permanent magnet, then if the relay accidentally receives a mechanical blow that can open the connection it is not clear how the ON state can be re-established. This latter problem, the ability of the ON state to be re-established after accidental opening, is in practical designs an important consideration.
U.S. Pat. No. 4,271,450 and U.S. Pat. No. 4,774,623 are examples of relays that use a bistable electronic control circuit, but these designs are difficult to implement in practical relay designs.
For the above reasons while bistable relays have a number of theoretical advantages, they have nor to date achieved widespread practical applications.
According to the invention there is provided an electromagnetic relay comprising, a stationary first member and a second member adapted to move towards and away from the first member such that when the second member moves towards the first member an electrical contact is closed, spring means for normally biasing said members apart, permanent magnet means for generating a force of attraction between said members, and selectively operable means for generating an electromagnetic force, wherein the permanent magnet means has a strength such that when the first and second members are apart the permanent magnet means is insufficient to overcome the spring means and wherein when the said members are brought together the permanent magnet means is able to hold the members together against the spring means, and wherein said means for generating an electromagnetic force can be operated in a first condition to provide an attractive force between said members sufficient with said permanent magnet means to overcome said spring means and to bring said members together, and in a second condition to provide a repulsive force between members sufficient with said spring means to overcome said permanent magnet means and move said members apart.
Preferably switch means are provided whereby the means for generating an electromagnetic force is switched off when the members are brought together. The switch means may be adapted to turn on the means for generating an electromagnetic force in the event of an accidental displacement of the first and second members.
The permanent magnet means is preferably provided on said movable second member, while the means for generating an electromagnetic force is provided on the fixed first member.
The first and second members may comprise a pair of U-shaped magnetic yokes, the arms of said yokes facing each other, with the permanent magnet means preferably being provided at the ends of the arms of one said yoke, and preferably the means for generating an electromagnetic force comprises a coil wound around one said yoke and means for supplying current to the coil. Means may be provided for selectively supplying current in opposite directions to the coil to generate the attractive and repulsive forces.
An electronic control circuit may be provided for supplying the current to the coil. This control circuit may include switch means for turning the relay on and off, and when the relay is turned on the circuit provides a current to the coil in a first direction to generate the attractive electromagnetic force and at the same time a capacitor is charged, and when said the is turned off the capacitor discharges to supply a current to the coil in the opposite direction to generate the repulsive electromagnetic force.